1. Field of the Invention
This invention relates to an image forming method, and more particularly to an image forming method of an electrophotographic system that is connected to a host computer such as personal computers, office computers, minicomputers or the like and prints images in accordance with image information and instructions sent from the host computer.
2. Related Background Art
In conventional image forming methods that employ electrophotographic systems such as digital copying machines and laser beam printers, commands relating to print and encoded character and design information are received as data from an external information processor, such as a computer or a work station, and the encoded information is converted into pixel information in a formatter. When converted, image data having density information such as in photographs are subjected to image processing such as dither matrixing and made into a binary form.
Next, this image information is printed at the part of an electrophotographic engine. An electrophotographic cartridge is comprised of an electrophotographic photosensitive member, a charging roller, a developing assembly and a cleaner as one unit. The electrophotographic photosensitive member comprises a cylindrical substrate made of aluminum or nickel and a photosensitive material such as OPC, amorphous Se or amorphous Si formed on the substrate, and is called a photosensitive drum. The surface of the photosensitive drum is uniformly electrostatically charged by the charging roller. Next, image signals are raster-scanned by a laser scanner. In the laser scanner, with the on-off of a semiconductor laser, image signals are scanned with a polygonal scanner to form optical spot images on the photosensitive drum by means of an optical system and a return mirror. Thus, an electrostatic latent image is formed. The electrostatic latent image formed is developed by the developing assembly. To carry out the development, jumping development, two-component development or feed development is used, where image exposure is carried out by turning on a laser at the recording area to make the latent image have no electric charges and is often used in combination with the reverse development carried out by causing toner to adhere to areas having less electric charges.
The image formed by development (a toner image) is transferred to transfer mediums. The transfer mediums are held in a cassette, and are fed sheet by sheet by means of a paper pick-up roller. Once print signals are sent from the host apparatus, a transfer medium is fed through the paper pick-up roller, and a transfer roller is operated while being synchronized with the image signals by a timing roller, so that the toner image is transferred to the transfer medium. The transfer roller is an elastic member having conductivity and a low hardness, where the toner image is electrostatically transferred by the aid of a bias electric field at a nip formed between the photosensitive drum and the transfer roller.
The transfer medium on which the image has been transferred is sent to a fixing assembly to fix the image, which is then delivered by a paper output roller and put out to a paper output tray. Meanwhile, to remove the toner remaining after transfer, the photosensitive drum surface is cleaned by a blade of a cleaner assembly.
The light-emission intensity and light-emission duty of the semiconductor laser of the laser scanner is controlled by an exposure control section.
Bias voltages applied to the charging roller, bias voltages applied to the developing assembly and bias voltages applied to the transfer roller are controlled by a high-voltage control section.
Main motors and scanner motors are controlled by a motor control section.
Pressure and temperature of the fixing assembly are controlled by a fixing control section.
The operation of the paper pick-up roller and timing roller is controlled by a paper feed control section.
In the conventional image forming method as described above, users have not so much been required to use graphic images as outputs from printers of an electrophotographic system, and where it is unnecessary to develop every dot with regard to character images and there has not been a problem. Recently, however, users often print out graphic images by the use of the electrophotographic printers, and it has become necessary to develop every dot. This is because, even though high resolution has been achieved as a result of an advance in scanner drivers, details may disappear at highlights and the valuable high resolution can not be well provided unless every dot is developed. Since, however, the on-state time of lasers becomes shorter and the laser spot diameter becomes larger with respect to the image size as the electrophotographic printers have a higher resolution, digital latent images can not be formed and the potential difference of a latent image (i.e., difference between dark portion potential Vd and light portion potential Vl of a latent image) becomes smaller, causing the problem that every dot can be developed only with difficulty. For example, when an image is printed using an electrophotographic printer having a resolution of 1,200 dpi (pixel size: about 21 .mu.m), and a semiconductor laser which emits light in such an intensity that the charge potential, (i.e., dark portion potential Vd) of an electrophotographic photosensitive member is -650 V and the light portion potential Vl of the electrophotographic photosensitive member is -150 V is used, the laser spot diameter of the semiconductor and the potential difference, .vertline.Vd-Vl.vertline. (V), of a one-dot latent image have the relationship as shown in Table 1 below.
TABLE 1 ______________________________________ Relationship between Laser Spot Diameter and Potential Difference of One-dot Latent Image Laser spot diameter One-dot latent image .vertline.Vd - Vl.vertline. (.mu.) (V) ______________________________________ 83 pm 63 62 pm 112 41 pm 224 20 pm 481 ______________________________________
That is, ideally, it is preferable for the laser spot diameter to have a size substantially equal to the size of one pixel. In practice, however, from the viewpoint of cost of optical systems, it is common for the laser spot diameter to have a size of about 80 .mu.m. The size of one pixel is about 84 .mu.m at 300 dpi, and about 42 .mu.m at 600 dpi, and hence, at 300 dpi, there is no problem when the laser spot diameter has a size substantially equal to the size of one pixel. At 600 dpi, however, the latent image is formed in a laser spot diameter having a size about twice that of one pixel. Nevertheless, although every dot can be developed if only the light intensity is controlled so long as the laser spot diameter can be within the size about twice that of one pixel, it becomes impossible to form the latent image digitally for every dot when, for example, the resolution is as high as 800 dpi or more, because the laser spot diameter has a size nearly three times the size of one pixel. Accordingly, in the electrophotographic printers, to perform the development of every dot necessary for obtaining graphic images with a high resolution, measures are taken such that the light intensity is made higher, the DC component of development bias is made stronger, or the dark portion potential Vd of a latent image is made higher.
In the above conventional case, it is true that every dot can be developed at a high resolution in the electrophotographic printers when measures are taken such that the light intensity is made higher, the DC component of development bias is made stronger, the frequency of development bias is made smaller, or the dark portion potential Vd of a lagent image is made higher, but another problem arises such that the images may block up at high-density areas of a gray scale. Such block-up of images at the high-density areas of a gray scale makes gradation poor at high-density areas of the graphic images, resulting in images not well provided with a high resolution and having a dark impression. It also brings about an increase in toner consumption to cause such a difficulty that fog may seriously occur correspondingly with an increase in the toner consumption. It still also results in an increase in the toner remaining on the drum after transfer, tending to cause the problem of faulty cleaning, and consequently the graphic images can not be improved even when formed at a high resolution.
Hence, basically the laser spot diameter must be reduced to the pixel size or so, but, if it is too much reduced, the focal length becomes not more than 1 mm, so that the scanner must be precisely fitted to the main body and the function of automatic focussing must be provided, resulting in a high cost.